It has been demonstrated that isometric relaxation is prolonged in senescent myocardium (Lakatta et al., J. Clin Invest. 55: 61-68, 1975) and that this is accompanied by a diminished rate of Ca ions accumulation in sarcoplasmic reticulum isolated from the senescent rat heart, when compared to the adult rat heart (Froehlich et al., J. Mol. Cell. Cardiol. 10: 427-438, 1978). The current investigation is designed to determine whether the slow relaxation may be related to hypertrophy of the senescent heart and whether or not relaxation rate may be modified by physical conditioning. In addition, the relationship between mechanical relaxation and sarcoplasmic reticulum function is explored utilizing perturbations of both systems, including chronic thyroid injections and catecholmines and then intracellular messenger equivalents.